1. Field of the Invention
The invention relates to a cold-cathode fluorescent lamp control circuit, and more particularly to a circuit for matching the current in multiple cold-cathode fluorescent lamps, especially for backlighting applications.
2. Related Art
The flat panel LCD market is rapidly growing, as consumers worldwide are adopting LCD televisions and other LCD devices. Manufacturers of LCD televisions are continuously increasing the screen size of these products. An LCD TV requires backlighting, which is typically achieved using several cold-cathode fluorescent lamps that are mounted inside the chassis directly behind the LCD display. The larger the screen size, the greater the number of lamps required.
A major problem with this method is that brightness gradients can appear across the screen due to uneven brightness levels in the respective lamps. FIG. 1 shows a typical CCFL circuit, including a conventional control circuit 100 such as the IR-2153 half-bridge driver, manufactured by the International Rectifier Corp. which controls the fluorescent lamp circuit at a given frequency.
As seen in FIG. 2, with the circuit of FIG. 1, when the resistances of the lamps are not equal, uneven current can flow in the respective lamps, causing uneven brightness levels in the various lamps. Standard CCFL lamps are not matched for voltage or current, and can vary due to normal manufacturing tolerances. Because these lamps typically have a very large resistance (>100K Ohms) and small current (<5 mA), any small difference in voltage, current, or equivalent resistance results in a large difference in brightness. This brightness difference is very noticeable, especially with larger screens when more lamps are used, and causes an uneven light gradient behind the LCD panel.
It is therefore desired to provide a solution that will provide the lamps with both constant and equal brightness levels for a uniform backlighting of the entire LCD screen.
One known solution is a transformer balancing circuit (FIG. 3) which uses transformers to maintain equal current flowing through the respective lamps. Another transformer balancing circuit of background interest is disclosed in U.S. patent application Publication No. 2005/0093472. These circuits are useful, but expensive and bulky due to the large number of balancing transformers needed, especially for larger LCD screens. A typical 16 lamp backlight configuration will need 16 transformers.
According to one embodiment of the invention, a method of matching the brightness of a plurality of lamps driven by an AC drive current may comprise the steps of: selecting a first one of said lamps having a lowest brightness; and reducing the AC drive current in a second lamp periodically so as to reduce the brightness of the second lamp to match that of the first lamp. According to another embodiment, a reference brightness may be selected, or optionally a reference AC current level, and the method may reduce the drive current periodically so as to set the lamp brightness in relation to the reference brightness or optionally the reference AC current level.
A circuit for driving a lamp with controllable brightness may comprise: a drive circuit for supplying an AC drive current to the lamp; and a reducing circuit for reducing the AC drive current in the lamp periodically so as to set the AC drive current in the lamp to match a reference. The reference may correspond to the brightness of another lamp driven by the AC drive circuit, or to a reference AC current level.
The reducing circuit may comprise a switch in series with the lamp, or a parallel connection of a resistance and a switching device connecting the lamp to ground. The AC drive current may be reduced by a predetermined number of half-cycles, preferably by one half-cycle, of said AC drive current. The AC drive current may be reduced by supplying periodic OFF pulses to the reducing circuit, which may be spaced apart by a selected number of half-cycles of the AC drive current, for example by a frequency-dividing circuit which receives and divides the frequency of the AC drive current.
Using an IC to perform the balancing results in a great improvement. Since the lamp currents are very low, the power consumption using an integrated circuit will be low.
Other features and advantages of the invention will be understood from the following description of an embodiment thereof, with reference to the drawings.